New 365 mya Tetrapod Limb Fossil

["Richard W. Travsky" <rtravsky@uwyo.edu>]

http://news.nationalgeographic.com/news/2004/04/0401_040401_tetrapodfossil.html

Today researchers announced their discovery of a 365-million-year-old
fossil limb bone of an ancient tetrapod. Tetrapods, including humans, are
four-limbed animals with backbones. The fossil was found during road
construction that revealed an ancient streambed.
...
The bonea humerus, or upper arm or forelimbis one of the earliest tetrapod
limb bones ever found. (Tetrapods today include amphibians, mammals,
reptiles, and birds, among others.)

The ancient bone shares features with primitive fish fins, but also has
characteristics of a true limb bone. It bridges the gap between fish and
amphibian.

"The transition wasn't all or nothing," said Ted Daeschler, a vertebrate
zoologist with the Academy of Natural Sciences in Philadelphia "It's not
that some animals were thrown on land. There were certainly other
functions intermediate." 

Daeschler and colleagues Neil Shubin and Michael Coates, paleontologists
at the University of Chicago, say the fossil bone offers a window onto
this intermediate stage.

The National Geographic Society Committee for Research and Exploration and
the National Science Foundation supported the scientists' research. The
trio describe the fossil in tomorrow's issue of the journal Science.

"This new humerus shows some primitive features that are lost in later
tetrapods but in this specimen have already begun to change their
orientation into tetrapod-like configuration," said Jennifer Clack, a
paleontologist at the University Museum of Zoology Cambridge, United
Kingdom. 
...
The humeruswhich the scientists say clearly belongs to a limbed amphibian
and not a lobbed-fin fishsuggests that the tetrapod was an animal that had
a powerful forelimb with a large area for the attachment of muscles at the
shoulder. Such a muscle is associated with the ability to perform a
push-up.

"The [newfound] humerus enables comparisons with fish that were not
possible until now," Shubin said. "There is a large crest on the lower
side of this humerusit is where the pectoral muscle would attach. This
same expanded crest is seen in fish." 

The presence of a place to attach a pectoral muscle in both fish and
tetrapods suggests that the ability to perform a push-up is ancient,
evolving first in fish and not in terrestrial animals as was originally
believed.

"The notion is that this movement is primitive," Shubin said. He believes
the movement first arose in fish that required the appendages to move.
That motion was akin to how creatures eventually used limbs to walk on
land, Shubin said.

According to the researchers, however, fish with limblike fins had no
intention of walking on land. Rather, they were adapting to their
environment. 
...